1. Technical Field
The present invention relates to a redispersible powder, its aqueous dispersion, a process for the preparation of the redispersible powder and the use of the powder and dispersion.
2. Background Information
The stabilization of aqueous polymer dispersions (latices) is known from the prior art. As opposed to polymer solutions, aqueous polymer dispersions are not thermodynamically stable systems. Therefore, generally dispersing agents having an interfacial stabilizing action are added to the dispersions. Normally they are water-soluble compounds either in the form of protective colloids or emulsifiers. Protective colloids have a stabilizing action due to a steric or electrostatic shielding action, whereas emulsifiers stabilize the polymer dispersion due to their amphiphilic structure. These stabilizers, which can also be used in emulsion polymerization, are conventional surfactants, i.e., water-soluble polymers, such as polyvinyl alcohol or polyvinyl pyrrolidone.
Besides stabilized, aqueous polymer dispersions, the powders obtainable as a result of drying these dispersions are of great significance. Due to their easy handling, easier, space-saving transportation, easier dosability and less expensive storage, redispersible powders are advantageous. Due to the fact that the dispersing medium “water” is everywhere readily available, the powder form is also desirable in this connection. The drying processes for obtaining the redispersible powder are, e.g., performed by means of freeze or spray drying using protective colloids, such as polyvinyl alcohol. A particularly advantageous process for drying aqueous dispersions is spray drying and then larger powder quantities can be produced. In this process, the aqueous dispersion is sprayed in a hot air flow and dewatered and preferably the drying air and sprayed, aqueous dispersion pass in parallel flow form through the drier and, if necessary, known drying aids can be concomitantly used.
A problem when using surfactants or protective colloids is coalescence, i.e. the undesired flowing together of the latex particles in the emulsion, so that it is no longer possible to obtain redispersible powders. This agglomeration to larger polymer secondary particles (coagulate), for a given polymerizate content of the aqueous dispersion, becomes all the more critical the more finely divided the dispersed particles, because the interface grows in super-proportional manner with decreasing particle diameter.
A corresponding powder should admittedly be completely reversibly dispersible, but on adding water generally completely satisfactory results do not occur. This is linked with the fact that on drying the highly dispersed particles in the dispersion necessarily approach one another until in the case of contact of the surfaces of the particles irreversible changes occur, such as the aforementioned coalescence or also an aggregation of the particles. As a result the surface characteristics of the disperse phase are so modified that on adding water, the affinity of the particles to one another is greater than that to water, so that there is no longer any real redispersion.
The aforementioned emulsifiers or protective colloids, such as, e.g., polyvinyl alcohol, can also give rise to a reduction in the reactivity of the redispersed polymer particles. In other words, the redispersible powder, following redispersion, partly or entirely loses its reactivity and consequently the characteristics associated therewith, so that the further reaction of the copolymerized, functional monomers, after dispersion, is greatly impaired or does not occur. One possibility for obtaining stabilized dispersions from water-insoluble latex is known from EP 441 037 A1. The latter describes anionically stabilized dispersions from latex and a quaternary, cationic polymer, preferably a polyamide epichlorohydrin resin, the two components being separately prepared and stored as soluble polymers. Both components are sprayed together onto the application surface, so as to form a dry coating which cannot be washed out, the cationic polymer serving as a flocculant. Therefore the cationic polymers do not contribute to the stabilization of the dispersion and instead destroy the latter, accompanied by the precipitation of the latex polymers (so-called demulsification) and the formation of a coating. No mention is made of redispersible powders.
EP 286 008 B1 describes the use of aqueous, cationic plastic dispersions for impregnating and priming absorbent substrates. The cationic dispersion polymerizates contain 80 to 99 wt. % ethylenically unsaturated monomers from the group vinyl esters, methacrylic esters, acrylic esters, vinyl aromatics, vinyl chloride, ethylene, acrylonitrile. diesters of maleic acid and/or fumaric acid, vinyl pyrrolidine, 1 to 20 wt. % ethylenically unsaturated, cationic, water-soluble monomers, 0 to 19 wt. % ethylenically unsaturated, hydrophilic monomers with one or more functional groups from the series COOH, —NR1R2, —CONR1R2, in which R1 and R2 stand for H or —CH2OR with R═H or (C1 to C8) alkyl and up to 19 wt. % ethylenically unsaturated monomers with one or more OH groups. The monomers are chosen in such a way that the minimum cationic activity of the dispersion is 20 to 200 μmole/g solids, measured at pH 7, and 60 to 99% of the cationic charge is on the surface of the particles and the dispersions have a minimum film formation temperature (MFT) between 0 and 40° C. The polymerizate particles of the dispersions have an average diameter of 0.02 to 0.2 μm. The cationic dispersions are used in impregnating and priming brickwork, concrete, plaster surfaces, ground plaster, gypsum surfaces or bricks. However, dispersible powders are not described.
JP 55-104 955 A describes an aqueous dispersion of a cationic, ethylenically unsaturated polymer having a glass transition temperature Tg<50° C. and containing a cationic, water-soluble or water-dispersible, ethylenically unsaturated oligomer and/or polymer and/or cationizable, ethylenically unsaturated monomers in the aqueous phase. The polymers serve as additives for improving the characteristics of cement products. As a result the water resistance, water tightness, strength, adhesion, chemical stability and durability of a cement product, such as mortar are improved. The emulsion polymerization of said monomers is either brought about by radical polymerization of alpha or beta-ethylenically unsaturated monomers in the presence of cationic, water-soluble and/or water-dispersible oligomers and/or polymers or by radical polymerization of alpha or beta-ethylenically unsaturated monomers, which are in part replaced by cationic, ethylenically unsaturated monomers, such as dimethylaminoethyl methacrylate esters of maleic acid, fumaric acid, itaconic acid, etc.
A disadvantage of this teaching is on the one hand the very restricted range of polymers which can be used, together with the very restricted field of use for cement formulations.
The polymers produced must have a glass transition temperature Tg<50° C. in order to give mortars the desired characteristics. Moreover, the cationic acrylic resin according to this Japanese patent serves as a water dispersing agent and is used as an additional additive. Thus, the use of the cationic, water-soluble or water-dispersible oligomer and/or polymer serves to prevent the coagulation of the latex in the cement. JP 55-104 955 A also fails to describe a redispersible powder, which is also not provided. For the above reasons it is much more difficult, in addition to a stable dispersion, to provide a redispersible powder obtainable therefrom.
JP 46-22922 describes a process for the preparation of cationic vinyl polymer emulsions, the monomer being a diene and/or vinyl compound, which is polymerized in the presence of a water-soluble, thermosetting, cationic resin. The resin is prepared by reacting water-soluble, unmodified or modified homopolyamide resins, which substantially only contain secondary amino groups, with epihalohydrins in water and use can also be made of a nonionic emulsifier. Emulsions prepared in this way are used for improving the strength, water resistance, thermal stability and adhesion of emulsion polymer films in paper manufacture and processing or also in cement formulations. JP 46-22922 does not describe redispersible powders. The very restricted range of starting materials used is disadvantageous here.